High current vacuum gap devices



UctV, 1969 J. A. RICH HIGH CURRENT VACUUM GAP DEVICES 4 Sheets-Sheet 1 Filed May 19, 1967 5a I r /n vemor Joseph A. Ric/7 by His Af/arney.

7, 1969 J. A. RICH 3,471,733

HIGH CURRENT VACUUM GAP DEVICES 7 Filed May 19, 1937 4 Sheets-Sheet f F I. w

/nvenf0r Joseph A. Ric/2 His Afro/nay.

@ci. 7, 1969 J. A. RICH 3,471,733

HIGH CURRENT VACUUM GAP DEVICES Filed May 19, 1967 4 Sheets-Sheet 3 as 84 83 a7 /nveni0r Joseph A Rich His Arrorney.

Oct. 7, 1969 J. A. RICH 3,471,733

HIGH CURRENT VACUUM GAP DEVICES Filed May 19, 1967 4 Sheets-Sheet 4 Fig. /0.

/n venfor Joseph A. Ric/7 His Afro/nay- US. Cl. 313-217 10 Claims ABSTRACT OF THE DISCLOSURE Vacuum are devices such as triggerable vacuum gaps and vacuum switches are formed with at least one primary arc-electrode in the form of a reentrant conductor so that arcing current flowing along the re-entrant path results in zero net magnetic field between the primary arc-electrodes. The other arc-electrodes is of such a configuration that it causes no net magnetic field within the interelectrode gap. In the absence of magnetic field, arc currents do not bunch up and form anode spots. Second arc-electrode may be made to have no magnetic field effect in the interelectrode gap by causing it to completely enclose the gap within its volume, or to be re-entrant itself or both.

The present invention relates to the copending, concurrently filed application of J. M. Latferty, Ser. No. 639,844, and my copending, concurrently filed applications, Ser. Nos. 639,834 and 639,843, assigned to the present assignee. The present invention relates to vacuum gap devices adapted to operate at high currents without the formation of anode spots therein. More particularly, the present invention relates to such devices in which the formation of anode spots is avoided by the configuration and position of the electrodes such that essentially no magnetic field exists Within the interelectrode gap.

In the development of vacuum switches and triggerable vacuum gap devices, a limiting factor to the amount of current which can be drawn by a given structure is the threshold current at which a destructive anode spot is formed. Formation of such anode spots results in erosion of the anode electrode and melting thereof. Such erosion and melting adversely affects the surface of the device, making the breakdown voltage change from its original value, eventually leading to failure of the device. In development of prior art vacuum gap devices, many expedients have been utilized in order to keep anode spots and cathode spots alike moving over the surface of the arc-electrode so as to avoid undue erosion and melting thereof. Although this is a useful technique and does in fact result in longer life for vacuum gap devices, it nevertheless only postpones eventual failure, because even a moving are spot or arc footpoint causes some melting and erosion of the arc-electrodes. In order that optimum resistance to erosion and melting be achieved, it is desirable that anode spots be completely eliminated from vacuum gap devices.

Accordingly, it is an object of the present invention to provide vacuum arc devices wherein the electrode configuration and position greatly increases the current threshold for the formation of anode spots.

Yet another object of the present invention is to provide vacuum arc discharge devices which are capable of carrying much higher currents than prior art devices without the formation of anode spots.

Still another object of the present invention is to provide vacuum arc discharge devices in which the interelectrode gap is substantially free of any magnetic field.

In accord with one embodiment of the present invention, I provide improved vacuum arc discharge devices having high current thresholds for the formation of anode spots and including a pair of primary arc-electrodes nited States Patent 3,471,733 Patented Oct. 7, 1969 "ice wherein at least one of the electrodes has a re-entrant form and current flow between the arc-electrode in the reentrant electrode causes a doubling back of the current path, resulting in cancellation of the net magnetic field due to current conduction by the re-entrant electrode. In further accord with the present invention, the other primary electrode is formed in such a manner so that it too has zero magnetic field to the interelectrode gap. This may readily be achieved by constructing the second arcelectrode in the form of an encompassing cylinder over the first primary arc-electrode so that, in accord with amperes law, there is substantially no net magnetic field within the interelectrode gap.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the following description, taken in connection with the appended drawings, in which:

FIGURE 1 is a vertical cross-sectional view of a triggerable vacuum gap constructed in accord with the present invention,

FIGURE 2 is a vertical cross-sectional view of a vacuum switch constructed in accord with the present invention,

FIGURE 3 is a vertical cross-sectional view of a portion of the electrode structure of an alternative embodiment to the device of FIGURE 1,

FIGURE 4 is a horizontal cross-sectional view of the electrode structure illustrated in vertical section in FIG- URE 3,

FIGURES 5 and 6 represent horizontal cross-sectional views of alternative structures to the structure illustrated in FIGURE 4,

FIGURE 7 is a vertical cross-sectional view of a device constructed in accord with the invention similar to the device of FIGURE 1, but with improved current carrying capabilities,

FIGURE 8 is a perspective view of an alternative device to that illustrated in FIGURE 7, and

FIGURE 9 is a horizontal cross-sectional view of the electrode structure of the device illustrated in FIG. 8.

FIGURE 10 is a vertical cross-sectional view of an improved alternative embodiment functionally equivalent to the device of FIGURE 7.

In vacuum arc devices, the current threshold marking the onset of the formation of anode spots is a function of electrode geometry and electrode material. For a given material, therefore, the formation of anode spots is a function of electrode geometry. In the plane-parallel geometry, frequently used in switches in general and vacuum switches in particular, the threshold is relatively low since a spot is formed at any point at which current density becomes high, either due to surface irregularities or anchoring of the are due to the interaction of electric and magnetic fields. One means of inhibiting the forma tion of anode spots is to use electrodes having a very large area, so that the currents between the arc-electrodes are diffused over a large area to prevent the formation of anode spots at low currents. Yet another means utilized is to cause any spot which has been formed, and its associated arc, to rotate by the interaction of electric and magnetic fields so that electrode burning and erosion at any given point is kept to a minimum.

In accord with the present invention, I increase the threshold current for the formation of anode spots by essentially eliminating the magnetic field within the interelectrode gap, so that currents therein are not caused to bunch together, thus substantially eliminating anode spots due to the high current densities. In most vacuum arc electrode devices, in the absence of an externally applied magnetic field to cause arc rotation, the are (other than that of the plane parallel electrode configuration) is acted upon by a force which is orthogonal to the current path between the electrodes and the magnetic field caused by the path of current through the arc-electrodes. In vacuum are devices, wherein large electrode areas are utilized, anode spots are caused by numerous conduction paths between the arc-electrodes being acted upon by the foregoing force, causing all of the conduction paths to move to a point which is a point of equilibrium at which the force tending to move the conduction paths is minimum. Accordingly, at the point at which the conduction currents become concentrated, a high current density results and an anode spot is formed. In accord with the present invention the bunching-up of conduction paths between the primary arc-electrodes is avoided, thus avoiding the formation of anode spots, except at exceedingly high currents.

In FIGURE 1 of the drawings, a triggerable vacuum gap includes an hermetically sealed envelope 11 including a pair of upper and lower end plates 12 and 13, respectively, connected together in hermetic seal by a substantially cylindrical insulating sidewall member 14. Sidewall member 14 is connected to end wall members 12 and 13 by appropriate metal-to-glass seals 15, as is well known in the art. Within envelope 11, a pair of primary arc-electrodes 16 and 17 are disposed from respective end-wall members 12 and 13 by upper and lower electrode support members 18 and 19. A first arc-electrode 17 is in the form of a re-entrant cup having a flanged outer cylindrical member 21 surrounding an inner re-entrant cup 20. A trigger device 23 comprising a scored hydride film, for example, on a ceramic cylinder, with one side of the hydride film connected to arc-electrode 17 and the other connected to a trigger electrode 25, is attached to a portion of flanged end piece 22 on electrode 17. Trigger device 23, may, for example, be the trigger assembly disclosed in Latferty application Ser. No. 564,132, filed July 11, 1966, and assigned to the present assignee. An outer, cylindrical, cup-shaped primary electrode 16 substantially completely encompasses the volume of re-entrant primary electrode 17 and defines therewith a hollow cylindrical annular interelectrode gap 26. A second trigger electrode 27 comprising a trigger electrode 28, similar to that of trigger electrode 23, is inserted within aperture 29 in the side of cylindrical outer electrode 16. The trigger electrode lead 30 of trigger electrode 27 passes through an aperture 31 in upper end wall member 12 and passes through an hermetic seal to exit from envelope 11. Likewise, trigger electrode 25 exits through an aperture 33 in lower end wall member 13 through hermetic seal 34.

In operation, gap device 10 is connected in series or parallel circuit relation with an electric load, as desired, and, when it is desired to have the device 10 switch from a non-conducting to a conducting condition, a pulse of ionized plasma is propelled into the interelectrode gap 26 by one of th trigger assemblies 23 or 27. If the device is connected with arc-electrode 16 as anode, the trigger assembly 23 injects a plasma into the arcing region, if arc-electrode 17 is connected as anode, trigger assembly 27 is utilized to trigger conduction. Upon the intiation of a breakdown by the injection of an electronion plasma into the interelectrode space by one of the trigger electrodes, numerous small current conduction paths are established between the exterior cup 21 of arc electrode 17 and hollow cylindrical arc-electrode 16. This discharge is represented by arrows A. This conduction characteristic causes a current path through electrode support member 19, upward along inner cup-shaped member 20, downward along outer cylindrical member 21, across the inter-. electrode gap and up along the outer portion of external arc-electrode 16 and through electrode support member 18.

In arc devices of the type illustrated in FIGURE 1 of the drawing, but without the folded back re-entrant structure of inner electrode 17, the azimuthal magnetic field due to the conduction currents upwardly through one arcelectrode and downwardly through another arc-electrode results in a magnetic field B,,. This magnetic field would exist in the interelectrode gap. The current at any given point within-the interelectrode gap is represented by the current density equalling I the radial current density. In such structures the force per unit volume of conducting fluid is:

This force is orthogonal to both the current density and the azimuthal magnetic field and, in a structure such as that illustrated in FIGURE 1, in the absence of re-entrant electrode portion 20, would result in a force being applied upward to force the arclets to rise upward to the points indicated as B, at which points bunching of the current would occur and anode spots would be formed.

In accord with the present invention, however, longitudinal current'in the outer cylindrical current conductor, arc-electrode 16, causes no net magnetic field to exist at the interior thereof, by amperes law. Similarly, because the current path directed upwardly in inner cylindrical conductor 20 of arc-electrode 17 is substantially equal to the current directed downwardly in outer cylindrical conductor 21 of arc-electrode 17, but in opposite direction, the interelectrode space 26 has substantially no net magnetic field due to current in arc electrode 17. Accordingly, there is substantially no body force acting upon minute current paths A therein and the current does not bunch up until exceedingly high currents are reached, thus effectively preventing anode spots forming upon the arcelectrodes.

In one device constructed in accord with the present invention, and substantially as illustrated in FIGURE 1 of the drawing, with arc-eelctrodes 16 and 17 formed from OFHC copper, with envelope 10 evacuated to a vacuum of 10 mm. of mercury, an outer electrode 16 having a diameter of 6" and a length of 9", the inner electrode 17 having an OD of 4%" and a length of 8" and defining a primary arc gap spacing of currents of 50,000 amperes were repeatedly carried without any trace of electrode melting or erosion.

FIGURE 2 of the drawing illustrates a vacuum switch constructed in accord with the present invention in vertical cross-section. Vacuum switch 40 comprises an evacuable envelope 41 including the walls of outer electrode member 42 and inner electrode member 43. Inner electrode member 43 comprises a central re-entrant cylindrical member 44 and an exterior cylindrical member 45. Cylindrical members 44 and 45 are joined together at the uppermost end by an annular cap member 46. Outer electrode member 42 is comprised of a fiat end member 47 and a hollow cylindrical sidewall member 48. Inner electrode 43 and outer electrode 42 are sealed together to form an hermetic seal to close evacuable envelope 41 by annular glass-to-metal seal 49. Electrical contact to arc-electrode 43 is made by connecting with threaded stud 50. Similarly, electrical contact is made to arc-electrode 42 by means of threaded stud 51. An electric discharge between inner electrode 43 and outer electrode 42 is made by means of starter electrode 52, which is electrically connected to outer electrode 42 in the cylindrical sidewall portion 48 thereof. Reciprocating motion to establish contact between the two electrodes is permitted by means of sylphon bellows 53. Contact piece 54 of starter electrode 52 is suitably curved or otherwise shaped so as to fit the configuration of exterior cylindrical member 45 of arc-electrode 43. Alternatively, a plurality of small contact fingers may contact different surface portions of cylindrical member 45. If an alternative embodiment is utilized, in which a flat portion of surface 45 is contacted, a fiat plate electrode may be utilized at 54. With a voltage applied between arc-electrodes 42 and 43, an electrical discharge therebetween is inititated by separating contact electrode from arc-electrode 43, by a reciprocating motion outwardly, caused by means to operate the starting electrode represented, by arrow 55, but not shown. Upon the initiation of an electric discharge by the opening of starter electrode 52, which is conveniently selected to be of a refractory material, as for example tungsten, molybdenum, or the like, an electric arc is struck between the starter electrode and th surface of the inner electrode sidewall member 45. The are rapidly spreads to the surrounding portions of sidewall member 48 of arcelectrode 42 and, because of the high vapor pressure of the material utilized as the electrode thereof (as for example copper, copper-beryllium alloys, copper-bismuth alloys and the like as are well known to the art) the arc very rapidly seeks the portions of electrode 42 which may serve as a source of conduction carriers for the creation of additional electronion plasma.

When the arc has spread and is diffused between the many surface points of arc-electrodes 42 and 43 the annular spacing 56 between member 45 of inner electrode 43 and member 48 of outer electrode 42 becomes the interelectrode gap. As with the device of FIGURE 1, due to the encompassing structure of cylindrical sidewall portion of arc-electrode 42 and the re-entrant configuration of arc-electrode 43, substantially no magnetic field exists in interelectrode space 56 and substantially no body force is exerted upon charge carriers moving from one electrode to another so that a difluse conduction may occur and no bunching, with the concomitant establishment of an anode spot occurs.

FIGURE 3 is a perspective view illustrated in vertical cross-section of an alternative embodiment of the invention which departs from circular symmetry. In FIGURE 3 an evacuable envelope 60 comprises outer electrodes 61, inner electrode 62, defining therebetween an interelectrode gap 68, and glass-to-metal seal 63 therebetween. A trigger electrode assembly 64 is located in the sidewall 65 of outer electrode 61. A second trigger electrode 69 is located in the flanged portion 67 of inner electrode 62. If electrode 62 is to be utilized as cathode, trigger assembly 69 may be utilized. If outer electrode 61 is to be utilized as cathode, trigger 64 may be utilized. If alternating voltages are to be applied both triggers 64 and 69 may be simultaneously pulsed in order to cause breakdown between arc-electrodes 61 and 62.

Alternatively, the structure of FIGURE 3 may be utilized as a vacuum switch by eliminating triggers 64 and 69 and installing a starter electrode at 64 in FIGURE 3, similar to starter electrode assembly 52 of FIGURE 2 of the drawing. Alternatively, the envelope structure of FIGURE 1 may be utilized to enclose the device of FIG- URE 3.

FIGURE 4 of the drawing illustrates in plan view the electrode configuration of the device illustrated in perspective in FIGURE 3. In FIGURE 4, outer electrode 61 and inner electrode 62 have an elongated semi-cylindrical, partial-parallel configuration with rounded ends and planar center portions. The interelectrode gap 68 between outer and inner electrodes is smallest in the flat plate portion to cause a concentration of the discharge between electrodes 61 and 62 to be concentrated thereat. If the device of FIGURES 3 and 4 is to be utilized as a vacuum switch a starter electrode which is reciprocable to initiate breakdown between arc-electrodes 61 and 62 may conveniently be located at 71. Since the inner electrode wall is flat at this portion, a flat starter contact may readily be used, as in the device of FIGURE 2. Alternatively, if the device is to be a triggerable vacuum gap device, triggers may be located at 70, 71 and also at 72 and 73.

FIGURES 5 and 6 illustrate plan sectional views of devices in accord with the present invention alternative to the structure illustrated in FIGURE 4. These alternative embodiments illustrate respectively in FIGURE 5 a rectangular cross-section; in FIGURE 6 an eliptical cross-section. In each of these figures, an outer electrode 61 and an inner electrode 62 define an interelectrode gap 68 therebetween, the thickness of which is relatively small in the planar sections as compared with the end sections so that, due to conservation of energy, electric discharges between the outer and inner electrodes 61 and 62 are confined to the plane-parallel portions of the electrode in FIGURE 5 where the arc length is shortest and to the most nearly parallel portions of the electrodes in FIG- URE 7. All structures satisfy the criterion that the end portions, where the arc is liable to bunch together to cause the formation of an are spot, are avoided and the distance between the arc-electrodes at this part is made very large in order to preclude any electrical discharge therebetween according to conservation of energy.

In the embodiments of the present invention illustrated in FIGURES 1 through 6, the outer arc-electrode and current conductor has been a cylinder and current is longitudinal in direction, resulting in no interior net field. In these embodiments the inner electrode and current conductor is re-entrant so that substantially no body force is exerted upon current carriers in the inter-electrode gap because of substantial cancellation of magnetic fields due to currents within the arc-electrode and conductor constituting the interior re-entrant cylinder. In accord with another feature of the present invention, I have found that it is possible to approximately double the current capacity of triggerable vacuum gap devices and vacuum switch devices in accord with the present invention without enlarging the volumes or dimensions of the devices in gross. In accord with this embodiment of the present invention, I utilize a doubly re-entrant structure. In accord with this embodiment, not only is the inner arc-electrode and conductor made re-entrant, but also the outer arc-electrode and conductor is made re-entrant to double the interelectrode gap area and thus permit a substantial doubling of the current carrying capacity of the devices.

One device constructed in accord with this embodiment of the present invention is illustrated in FIGURE 7 of the drawing. In FIGURE 7, a triggerable vacuum gap device comprises an inner re-entrant arc-electrode 81, comprising a flat end piece 82, an inner cylindrical reentrant member 83, an outer cylindrical member 84, and an annular member joining cylindrical members 83 and 84 at the upper portion thereof. An annular flange member 86 extends outwardly from the lower portion of cylindrical portion 84. A second, outer arc-electrode 87 comprises an outer cylindrical sidewall member 88 havmg a flanged outer edge 89, a first re-entrant inner sidewall member 90 and an annular flat portion 91 joining the upper edges of cylindrical members 88 and 90. A third cylindrical sidewall member 92 completes the active portions of outer electrode 87 and is capped with a flat disc member 93. Contact is made to outer electrode member 87 at stud 94 and connection is made to inner electrode member 81 at stud 95. The flanged members 86 of inner electrodes 81 and 89 of outer electrode 87, respectively, are connected by an annular glass seal 96 to form an hermetic, insulating seal so that the interior of the device may be evacuated, as for example to a pressure of 10- mm. of mercury, for vacuum operation. The structure of the device 80 defines a first, outer interelectrode gap 97 between outer cylindrical member 88 of outer arc-electrode 87 and the cylindrical member 84 of inner electrode 81. A second interelectrode gap 98 is defined by cylindrical member 83 of inner electrode 81 and cylindrical member 90 of outer electrode 94. As in the dev ce of FIGURE 1, current is directed within the in dividual arc-electrode member portions as follows. In outer cylindrical member 88 of outer re-entrant arc-electrode 87 current is directed upwardly for example. In cylindrical member 90 of outer electrode 88 current is then directed downwardly. In cylindrical member 92 of arc-electrode 87 current is then directed upwardly. Current in inner electrode 81 is then as follows. In cylindrical member 83 current is directed upwardly; in cylin drical member 84 of electrode 81 current is directed downwardly.

As in the device of FIGURE 1, because of amperes law, current directed upwardly in exterior member 88 causes no net magnetic field on the interior thereof. Similarly, the downwardly directed current in member 84 and the upwardly directed current in member 83 substantially cancel exterior of the couple, so that the net magnetic field existing in the outer interelectrode space 97 is substantially zero. Likewise, due to the same criteria, the net magnetic field in inner interelectrode gap 98 is substantially zero. It will be appreciated, however, that the magnetic forces do not identically cancel and that there is a. slight residual force directed upwardly in gap 97 and downwardly in interelectrode gap 98. There is, however, no bunchiug of the paths of conduction carriers in these regions, since the fields are very weak as compared with the current conduction paths and the spacings between the inner electrode 81 and the outer electrode 87 at the upper portions of the gap is made substantially larger than the paths along the vertical portions of the interelectrode gaps so that conservation of energy keeps the current conduction paths from being forced by any residual J XB force from causing a bunchiug of the current conduction paths at the upper ends of the interelectrode gaps.

As with the device of FIGURE 1, conduction between arc-electrodes 81 and 87 of triggerable vacuum gap 80, to cause the device to switch from a non-conducting to a conducting state, is initiated by causing a trigger pulse of electric energy to be supplied to trigger assemblies 100-, located in flanged member 86 of arc-electrode 81, and 101 located within sidewall member 88 of outer electrode 87, respectively. These trigger assemblies are substantially the same as those illustrated in FIGURE 1 of the drawing and are triggered under similar conditions. Since the device of FIGURE 8 includes a doubly re-entrant structure and defines two coaxial interelectrode gaps, additional triggers may be supplied at 102 and 103, if desired. These triggers would have the same construction and would be pulsed in tandem with triggers 101 and 102, respectively, or simultaneously with all, if alternating current voltages were utilized.

Device 80 of FIGURE 7 may be utilized as a vacuum switch by replacing trigger 101 and triggers 102 and 103 if desired, with reciprocable starter electrodes as illustrated in FIGURE 2 of the drawing, wherein the structure of FIGURE 1 is adapted for use as a vacuum switch. As in the device of FIGURE 1, the starter electrodes would be initially in contact with the inner electrode 81 and a burst of electron-ion plasma would be introduced into the inter-electrode gap by reciprocably withdrawing the starter electrodes therefrom to cause an initial arc to be initiated which would cause material from the arcing arc-electrode to be vaporized and ionized, causing the electric discharge to spread throughout inter-electrode gaps 97 and 98 to cause a high current to be conducted between arc-electrodes 81 and 87 without increasing the current density at any portions thereof. Due to the fact that there is substantially no body force acting upon individual current conductors between arc-electrodes 81 and 87, current is conducted between all juxtaposed parallel portions of arc-electrodes 81 and 87 without any bunchiug due to a body force, which in turn is due to the interaction of the azimuthal magnetic field and the radial current. This is due to the substantial elimination of any azimuthal field by the cancellation of magnetic forces due to the reentrant structure of electrodes 81 and 87.

In accord with the present invention, the device 80 of FIGURE 7 may be constructed so as to depart from circular symmetry and provide flat plate electrode surfaces substantially as is done with the devices of FIGURES 1 and 2, as illustrated in FIGURES 3 and 4 of the drawing. Such a modification is illustrated in FIGURE 8 of the drawing.

FIGURE 8 illustrates in perspective, with a vertical cross-section therethrough, a triggerable vacuum gap 110 having an inner re-entrant arc-electrode 81 and an outer re-entrant arc-electrode 87 with sidewall members 83, 84, 88 and 90, and interelectrode spaces 97 and 98. As with the device illustrated in FIGURE 3, arc-electrode members 83, 84, 88 and 90 have a plane parallel relationship except at the ends thereof whereat the device is terminated in a curvature which renders the interelectrode spacings much larger than the interelectrode spacing between the electrodes in the plane parallel section of the device. This ensures that the electric discharge bet-ween adjacent portions of the arc-electrode is confined to the plane parallel portion thereof by conservation of energy, to preclude the bunchiug of conduction paths at the end portions of the interelectrode gap to preclude the establishment of anode or cathode spots thereat.

FIGURE 9 of the drawing is a plan view of the modified device of FIGURE 8, as illustrated in perspective in FIGURE 8. As is readily evident from the device of FIGURE 9, the end portion of the interelectrode gaps 97 and 98 are much greater than the plane parallel sections thereof to preclude bunchiug of the conduction paths thereat. FIGURE 10 illustrates an alternative to the device of FIGURE 7 in which the re-entrant electrodes are not closed.

From the foregoing it may be seen that the generalized concept of the present invention is the substantial elimination of anode spots in vacuum are devices such as triggerable vacuum gap devices and vacuum switches. This is opposed to the normal means for allowing for high current operation of such devices which accepts the fact that such spots do form and use various techniques to move the are, thus minimizing electrode erosion and deterioration.

In accord with the present invention I accomplish the elimination of anode spots at higher currents than has been obtained heretofore by substantial elimination of azimuthal magnetic fields which may act upon current paths by utilization of re-entrant electrode structures to cause substantial cancellation of external magnetic fields.

Thus because F =B J I minimize F by minimizing B In accord with the present invention such re-entrant structures may be utilized with one or both electrodes with substantially the same-result. The structure utilizing two re-entrant electrodes is preferred due to the increase current capacity attained thereby.

Although only several preferred embodiments of the invention have been shown and described, it isto be appreciated that many other configurations and structures are possible in the practice of the invention.

While the invention has been set forth herein with respect to certain embodiments thereof, many changes and modifications will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A gaseous vacuum discharge device adapted to carry high currents without the formation of anode spots comprising:

(a) An evacuable envelope evacuated to a pressure of 1O- torr or less (b) A pair of primary arc-electrodes therein in close juxtaposition to one another defining an interelectrode gap therebetween and adapted to sustain therebetween a high current electric discharge (b said arc-electrodes having a configuration in which at least one of said arc-electrodes has a reentrant structure and current flow therein results in substantially no net magnetic field in said interelectrode gap to co-act with arcing currents therein and cause said arcing currents to bunch together to form a high current density anode spot,

(c) Means for causing an electric arc breakdown to be established between said primary arc-electrodes, and

((1) Means for connecting said arc-electrodes in circuit with an electric load.

2. The device of claim 1 in which the device is a triggerable vacuum gap and the means for causing an electric breakdown to be established between said electrodes is at least one trig er assembly adapted to inject an electronion plasma in the interelectrode gap.

3. The device of claim 1 wherein the device is a vacuum switch and the means for causing an electric breakdown between the said arc-electrodes is a starter electrode in contact with one arc-electrode and adapted to make and break contact with the other are electrode.

4. The device of claim 1 wherein one arc-electrode is a re-entrant cylinder which produces substantially no net azimuthal magnetic field at the exterior thereof.

5. The device of claim 4 wherein said other arc-electrode is a hollow cylinder surrounding the re-entrant cylindrical arc-electrode and current therein results in substantially no magnetic field in the gap between said arcelectrodes.

6. The device of claim 5 wherein said cylinder has a right circular cross-section.

7. The device of claim 5 wherein at least one portion of said re-entrant cylindrical arc-electrode has a planar surface and a starter electrode is in electrical contact with said other arc-electrode and has a planar surface adapted to make and break contact with the planar surface of said one arc-electrode.

8. The device of claim 1 wherein both of said arc-electrodes have re-entrant structure and result in substantially no net magnetic field in the gap between said arcelectrodes.

9. The device of claim 1 wherein the current in adjacent portions of said re-entrant arc-electrode is in opposite directions and the magnetic field due to said current is substantially zero externally of said portions.

10. The device of claim 8 wherein both of said reentrant arc-electrodes current in adjacent portions of the same electrode is in opposite directions and the external magnetic field due to said current is substantially zero external to said portions.

References Cited UNITED STATES PATENTS 3,356,893 12/1967 Lafferty 315-111 3,356,894 12/1967 Latferty 315-111 JAMES W. LAWRENCE, Primary Examiner RAYMOND F. HOSSFIELD, Assistant Examiner US. Cl. X.R.

3l3--l98, 231; 3 l5-lll 

